Field of the Invention
This invention relates in general to circuit breaker trip arrangements and more particularly to improved or more economical apparatus utilizing a current limiter assembly for operating a circuit breaker trip crossbar.
Summary of the Prior Art
Several conventional techniques are used in circuit breakers for sensing the presence of overcurrents. One is the use of a magnetic assembly for responding to the magnetic field generated by the overcurrent to operate a trip bar. Another is the use of a bimetal which is deflected by the heat of the overcurrent to operate the trip bar. The operated trip bar in turn releases the operating assembly of the circuit breaker to open the main contacts of the breaker.
In circuit breakers having a relatively low ampere rating the bimetal is actually in the circuit extending through the breaker and for circuit breakers having high ampere ratings the bimetal is indirectly heated by attachment to a portion of one of the circuit elements commonly known as a heater, which increases in temperature as the current increases. In the latter case current does not pass directly through the bimetal.
Both systems require a relatively high resistance either in the resistance of the bimetal itself or by the circuit element to which the bimetal is attached in order to provide sufficient thermal energy to deflect the bimetal. This resistance effect can cause the breaker to exceed the allowable or safe temperature rise even if carrying a rated current.
In U.S. Pat. No. 3,943,473 and related patents, a current limiter assembly for a circuit breaker is disclosed utilizing a pair of current limiter contacts in series with the main contacts of the circuit breaker. The current limiter assembly includes an electromagnetic assembly having a low resistance winding in series with the circuit breaker contacts and which responds to extremely high fault currents for separating the current limiter contacts. In this arrangement no means were provided under direct control of the current limiter assembly for ensuring that the main contacts of the circuit breaker were opened by the magnetic assembly responsive to high fault currents.